Itaconic acid (“IA”), in high demand in the chemical industry, is a precursor compound commonly used in manufacture of various products, such as acrylic fibers, rubbers, artificial diamonds, and lens. Certain filamentous fungi (e.g., Ustilago, Helicobasidium, and Aspergillus) converts monosaccharide to this compound. It has been found that cis-aconitic acid decaroxylase (“CAD”) plays a key role in the biosynthesis of IA.